1. Field of the Invention
The present invention relates to an exhaust gas-purifying catalyst including oxygen storage material.
2. Description of the Related Art
Until today, as an exhaust gas-purifying catalyst that treats exhaust gas of an automobile, etc., a three-way catalyst with precious metal supported by a refractory carrier made of an inorganic oxide such as alumina is widely used. In the three-way catalyst, the precious metal plays the role in promoting the reduction of nitrogen oxides (NOX) and the oxidations of carbon monoxide (CO) and hydrocarbons (HC). Further, the refractory carrier plays the roles in increasing the specific surface area of the precious metal and suppressing the sintering of the precious metal by dissipating heat generated by the reactions.
Jpn. Pat. Appln. KOKAI Publication No. 1-281144, Jpn. Pat. Appln. KOKAI Publication No. 9-155192 and Jpn. Pat. Appln. KOKAI Publication No. 9-221304 describe exhaust gas-purifying catalysts using cerium oxide or an oxide containing cerium and another metal element. These oxides are oxygen storage materials having oxygen storage capacity. When an oxygen storage material is used in a three-way catalyst, the above-described reduction and oxidations can be optimized. Even for a three-way catalyst using an oxygen storage material, however, it is difficult to achieve an excellent performance in both the state just after starting an engine and the state the engine is driven continuously as will be described below.
In the state just after starting an engine, the temperature of the catalyst is low. The ability of a precious metal to purify an exhaust gas in low temperature conditions is lower than the ability of the precious metal to purifying the exhaust gas in high temperature conditions. Thus, considering the performance just after starting an engine, decreasing the heat capacity of the exhaust gas-purifying catalyst is advantageous.
On the other hand, in the state where the engine is driven continuously, the temperature of the catalyst is sufficiently high. In this case, since the ability of the precious metal to purify the exhaust gas is high, it is advantageous that the exhaust gas-purifying catalyst to contain much more amount of oxygen storage material in order to respond to fluctuations of the exhaust gas composition.
As above, the performance just after starting an engine and the performance in the state where the engine is driven continuously are in a trade-off relationship. For this reason, achieving an excellent performance in both the state just after starting an engine and the state where the engine is driven continuously is difficult, and therefore consistently achieving a high exhaust gas-purifying efficiency is difficult.